The present invention relates to a valve gear of an engine for vehicles or the like, and particularly to a valve gear in which cams operative to control opening/closing of a valve are switchable.
A valve gear of an engine, in which plural cams having different-shaped nose portions are provided for each valve, and the valve-opening amount, the valve opening-closing timing, and the like are configured to be changeable according to an engine's operation state through a selection of a specified cam for opening/closing the valve from the plural cams, is known.
Japanese Patent Laid-Open Publication No. 2013-083202 and US Patent Application Publication No. 2011/0226205 A1, for example, disclose that a valve gear, in which a camshaft is comprised of a shaft portion and a cylindrical cam element portion which is coupled to the shaft portion with spline coupling so as to be moved in an axial direction of the shaft portion, the cam element portion has, at its outer periphery, plural cams for each valve which have different-shaped nose portions provided adjacently to each other, and a cam for opening/closing the valve is configured to be switchable through a move of the cam element portion in the axial direction.
Herein, in the valve gear disclosed in the above-described patent documents, a pair of end-face cams are provided at both end faces of the cam element portion and there are further provided a pair of operational members, each of which is configured to project to a position facing the corresponding end-face cam and contact this end-face cam so as to move the cam element portion, in the axial direction, toward an arrangement side of the other operational member or retreat from the above-described position facing the corresponding end-face cam. The above-described operational members are driven (projected) by actuators, so that switching operation of the cams can be conducted. Each of the above-described end-face cams has a lift portion which is configured to project in the axial direction such that the amount of projection of the lift portion increases gradually along a rotational direction of the cam element portion and a descent portion which is configured such that the amount of projection thereof decreases gradually along the rotational direction of the cam element portion.
There is further another conventional valve gear show in FIG. 12, in which an end-face cam 123 of a cam element portion 120 has a step portion 123a which is configured such that the amount of projection, in the axial direction, thereof decreases suddenly, in place of the above-described decent portion.
Meanwhile, it may happen that an engine equipped with the above-described valve gear rotates reversely, which is caused by a reaction force at a compression stroke of the engine, when the engine stops abruptly right after the cranking or when the engine is in a stop state. Therefore, there is a concern for the valve gear of the above-described patent documents that if the engine rotates reversely in a state in which the operational member projects, this operational member contacts the decent portion of the end-face cam of the reversely-rotating cam element portion, thereby moves the cam element portion in the axial direction, so that the cam element portion may be switched unexpectedly and improperly.
Further, in the case of the valve gear shown in FIG. 12, if the engine rotates reversely (in an arrow Y direction) in a state in which the operational member 132 projects, there is a concern that the operational member 132 hits against the step portion 123a of the end-face cam 123 of the cam element portion 120, so that the operational member 132 may break down.